This application claims priority from Korean patent application 99-32546 filed Aug. 9, 1999 in the name of Samsung Electronics Co., Ltd., which is herein incorporated by reference.
Semiconductor devices utilize output circuits called off-chip driver circuits for transmitting signals from the semiconductor device to other devices though transmission lines such as those formed on printed circuit boards. An example of an off-chip driver circuit is disclosed in U.S. Pat. No. 5,440,258. For optimum performance, the output impedance of the off-chip driver should be matched to the impedance of the transmission line. Therefore, semiconductor devices often utilize variable impedance control circuits to control the impedance of off-chip driver circuits. A variable impedance control circuit typically utilizes an extra pin to adjust or xe2x80x9ctrimxe2x80x9d the impedance of the off-chip driver circuit.
For example, in a semiconductor device having a high speed transceiver logic (HSTL) interface, a variable impedance control circuit senses the value of a resistor connected to the extra pin and trims the impedance of an off-chip driver circuit responsive to the value of the resistor. In such a semiconductor device, the main power supply voltage VDD is typically applied to the main circuitry of the device (e.g., a memory cell array) while a reduced power supply voltage VDDQ is applied to the output circuitry.
However, because the supply voltage VDD is used as the operational voltage for driving the variable impedance control circuit and the off-chip driver circuit, the impedance control circuit and the off-chip driver circuit are susceptible to changes in the level of the supply voltage VDD, thereby causing impedance mismatches.
One aspect of the present invention is a variable impedance control circuit comprising: an impedance matching transistor array operated by a first supply voltage; and an array driver coupled to the impedance matching transistor array, wherein the array driver includes one or more first internal elements operated by the first supply voltage and one or more second internal elements operated by a second supply voltage.
Another aspect of the present invention is a semiconductor device comprising: an off-chip driver circuit having a transistor array operated by a first supply voltage; and an output buffer having an array driver coupled to the transistor array, wherein the array driver includes one or more first internal elements operated by the first supply voltage and one or more second internal elements operated by a second supply voltage.
An additional aspect of the present invention is a variable impedance control circuit comprising: an off-chip driver having first and second transistor arrays; an array driver coupled to the off-chip driver and adapted to drive the first and second transistor arrays responsive to control code data; and a control code data generator coupled to the array driver and adapted to generate the control code data, wherein the control code data generator is operated from a first supply voltage; wherein the array driver includes drive elements coupled to the first and second transistor arrays, and the drive elements and the first and second transistor arrays are operated from a second supply voltage.